[Feature Request] autolevels - automatic R/G/B color correction curves to fix complex slide film color casts

[radioproektor]

What if add to NegPy Slide Mode tool named "autolevels"? It is a small AI model specially trained to draw "classic" R/G/B color correction curves to fix complex slide film color casts in blacks/shadows/midtones/highlights/whites independently. Currently It is available as source, as standalone app RetroShine, and as Lua plugin for darktable.

https://github.com/yellowdolphin/autolevels
https://github.com/yellowdolphin/retroshine
https://retroshine.eu/index.html

[thetalkingdrum]

This looks interesting, much like Kodak/ASF's old Digital ROC plugin, which came out of the same family as Digital ICE. The source is GPL, so licensing shouldn't be an issue.

[radioproektor]

I can say that works really well and saves a lot of time. I helped to improve RetroShine and done a lot of tests with developer, so most kindergarden problems and color management bugs where fixed in last version. AI model was trained on sRGB images, but autolevels/RetroShine now creates internal gamma adaptation so image with any input gamma/TRC/color space now corrected perfectly.
The only possible problem that AI model designed in the way that clips black and white to 0%-100% and this is not always desirable.
So i suggested to developer in future version add an option to manually adjust black and white clipping level, but apply it after automatic curves correction. If AI processing is done in 32 bit, it should be no problem to recover some clipped (negative values) data from blacks and whites. In this case AI model will work as usual and will set normal black and white points.
This user setting will help to make exported images less contrast and more friendly for additional manual tonal correction. In image editors this setting usually named "Output Levels".

Maybe something similar could be done in NegPy itself.

[radioproektor]

As an alternative to this automatic tool it could be a simple curves-based manual tool with multiple gray balance color picker eyedroppers.
So for example you click to shadow, midtone, highlight and each point aligns r/g/b curves but don't delete earlier created points.
Or it could be 3-5 separate color pickers.
Something similar to NegPy Highlights Shadows WB in negative mode but Not a WB based, but Curves based.
One of the main problems with old slides and negs is randomly faded shifted colors that always require this complex correction sometimes up to 5 points. And the fastest way to do it to use multiple gray color picker eyedroppers.

I asked similar specially for negative and slide correction in this thread there are a lot of different ideas there and in the end explained pros and cons about single vs multiple eyedroppers https://www.pl32.com/forum3/viewtopic.php?t=6918

[thetalkingdrum]

I was a bit quick to react. There are some problems with not all parts of that tool being GPL that might make it difficult to add. But Digital ROC did this 25 years ago with no AI, so it should be possible to make something similar, somehow. I think even VueScan has an implementation for faded dyes that works quite well.

[radioproektor]

Well i guess Digital ROC plugin was more like automatic multiple gray picker that search areas closest to gray and fix them. By the way i remember that Digital ROC also always clipped black point to pure 0% so i didn't like that.
But RetroShine, as it was explained by developer, detects objects (trees humans) and made corrections based this detection.

[radioproektor]

So if there is a problem with RetroShine, maybe just add multiple manual gray pickers?

[marcinz606]

do you have some example scans of those faded slides? at least for those that I managed to gather the negpy normalization (when turned on) seems to handle it pretty well. I'm not interested in aping some other software

[radioproektor]

Here is folder with original scan and ICC profile https://drive.google.com/drive/folders/1LHTLK_gyuuUr7FyxypT8LWyJ5d246sST?usp=sharing

Here is original image and processed using RetroShine (no any manual adjustments).

Here is how image opened in NegPy (have no idea why it is always looks so bright there. Another color management bug?):

Here how it looks if i click is Normalize button. Something totally wrong:

P.S. Color management set like this:

[marcinz606]

I'm against packaging "black box" models into negpy as I want the print pipeline explainable and deterministic. I have enough ai in my day job :)

[marcinz606]

Multiple pickers and arbitrary curves are also big nogo, it's agains the whole idea I had for negpy. At this point you might as well use darktable / lightroom or any other standard raw editor.

[thetalkingdrum]

I started to work on a feature last night based on expired kodak patents and some other open publications:)

Got some super early prototype running, but I'm not sure how far I will be able to take it yet, and I also do not have a lot of faded slides to test it on.

They seem to all work on the density level, so I could reuse parts of the density mixer we already have.

Sources:

Kodak, Restoration of faded images, US 5,796,874 (Woolfe, Pillman, Barry; filed 1996, expired) — https://patents.google.com/patent/US5796874

Daniell's PhotoRestore (white papers in White Papers/) — https://github.com/stongey/PhotoRestore

Chatterjee, Trumpy & Ruedel (2023), Digital Unfading of Chromogenic Film Informed by Its Spectral Densities, Heritage 6(4):3418–3428 — https://doi.org/10.3390/heritage6040181

[marcinz606]

I enabled cast removal slider for slides (it was gated to color positive mode before) on main branch earlier today. Cranking it up to max on this slide seems to make it noticeably better